In the field of ophthalmology, for example, intraocular pressure is measured to diagnose certain ophthalmological pathologies, with the main pathology being glaucoma of the eye.
Various devices are in existence at present for measuring intraocular pressure, and they can be classified in two main categories: tonometers and devices for measuring by means of laser interferometry.
Contact tonometers comprise essentially indentation tonometers, such as the Schlötz tonometer, and applanation tonometers, of which the most widespread is Goldmann's tonometer.
Indentation tonometers use a piston to deform the wall of the eye by pressing into the cornea, with IOP being measured by measuring the distance traveled by the piston. The major drawback of indentation tonometry is that account needs to be taken of the stiffness of the eye wall, and this leads-to variability of measurement from one eye to the other. That is why it can be assumed that that measuring technique is no longer in use nowadays.
Applanation tonometers use the known principle whereby the pressure which exists inside a spherical pressure chamber such as the eye has a relationship with the force which is capable of flattening a certain area of the sphere.
More particularly, amongst applanation tonometers, Goldmann's tonometer comprises a flattening cone made of plastics material containing a biprism which transforms the round image of the flattened cornea into two semi-circles which coincide when the cornea is flattened. The flattening cone is connected by a rod to a system using a calibrated spring to generate the force needed for flattening and which converts said force into millimeters of mercury (mmHg). The main drawback of that tonometer is that it is traumatizing and painful when used on the eye since a mechanical deformation force is applied to the cornea. As a result, repeated measurements on the same eye can lead to a lesion of the corneal epithelium, and in practice that method must necessarily be implemented by an ophthalmological doctor with the eye being anesthetized locally by means of anesthetic eyedrops. In spite of providing satisfactory sensitivity, there are also numerous causes of error in measurement: variability associated with tears, with accommodation, with thickness of the cornea.
A second type of applanation tonometer, also referred to as a “contactless” tonometer, uses a short puff of air to deform the cornea so as to make it concave, thus passing through a stage in which the flattened surface of the cornea presents a best angle of reflection between the light source and an optoelectronic sensor; it is this maximum which is detected and considered as the moment of measurement. The time that elapses between the beginning of the puff of air and maximum reflection on the cornea can be converted into a value for intraocular pressure. That type of tonometer likewise requires harmful mechanical deformation of the cornea.
More recently, devices have been proposed for measuring intraocular pressure by laser interferometry, with the major advantage of avoiding any application of a mechanical pressure force that is exerted on the cornea in order to deform it while making the measurement. Such devices rely on setting the eye into vibration, for example by means of a soundwave, and on using a Michelson type interferometer to detect the frequencies of natural modes of vibration of the eye. Work has shown that there is a simple relationship between the frequencies of the natural modes of vibration of the eye and intraocular pressure.
An example of that type of device is described in international patent application WO-A-93/21820. In that type of interferometer, the main incident laser beam is split by a splitter mirror (referenced 46 in the embodiment shown in FIG. 2 of WO-A-93/21820) into two secondary incident beams that are oriented at 90° to each other, one of the two secondary incident beams is reflected by the surface of the cornea of the eye, while the other secondary incident beam being reflected by a mirror (referenced 70 in the embodiment of FIG. 2 of WO-A-93/21820). The two return beams as reflected respectively by the cornea and by the mirror return to the splitter mirror where they interfere. A major drawback of that device is that in order to obtain interference, it is essential for the return beams to be accurately colinear. As a result, the cornea must be accurately in alignment relative to the splitter mirror, and it is not possible to accept the slightest lateral or longitudinal misalignment of the cornea, which makes taking a measurement very constraining in terms of eye positioning. Another result is that that device is extremely sensitive to the slightest external disturbances which might modify the alignment of the optical system very slightly, and in particular it is very sensitive to the slightest movement or mechanical vibration of the device. Another drawback of that type of device is that detecting the natural frequencies of vibration of the eye by measuring the intensity transmitted by the interferometer makes it necessary, in practice, to use a high power laser, with the risk of creating a lesion of the cornea. Finally, in international patent application WO-A-93/21820, the eye is excited in order to set it into vibration using a harmonic method by sweeping the frequency of the excitation soundwave. That harmonic excitation method presents the drawback of increasing the length of time required for measurement, and above all the soundwave may be traumatizing for the eardrum.